REST API: Install plugin translations after the plugin install. This only...

REST API: Install plugin translations after the plugin install. This only installs for the plugin in question, not all plugins.

Support for retrieving the langauge pack alongside the install API request was added in https://meta.trac.wordpress.org/changeset/10091 to avoid having to make a plugin update check during the REST API check.

Fixes #50732.
Props dd32, ocean90, ryelle, swissspidy, tellyworth, whyisjake, TimothyBlynJacobs.

Built from https://develop.svn.wordpress.org/trunk@48641


git-svn-id: https://core.svn.wordpress.org/trunk@48403 1a063a9b-81f0-0310-95a4-ce76da25c4cd